Just a few thoughts on your post
There are obviously Multi IR positions, but as they are better paid there will be competition from well qualfied applicants. Many Multi IR instructors have been flying air taxi or line flying and have gone back to instructing, when operations have folded.
I think your estimate for traing costs is too low. Unless you have access to a cheap twin and a cheap multi instructor you will blow the budget on the training alone. You will also need two flight tests with the associated aircraft and examiner costs.
As you have no applied instructional experience, you will be at a disadvantage. You should also consider the other kind of questions any future employer will look for on your CV. Consider your level of X-country IFR experience, in IMC? single-pilot? in airways? Have you done any instruction in a simulator? This is quite a common requirement.
A suggestion might be to get the Applied IF removed so you can teach the IMC. Having an IR when teaching the IMC in some people's opinion is more desirable. You should also earn more money. Why not consider removing the Night Flying restriction? At this time of year its not much use, but come the autumn. Night instruction can also pay more.
there is no further CAA requirement once you have the qualifications. However there will be standardisation training to ensure you teach the way the school requires.
The relevant sections are at the following link
Best of luck